Re: [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages

On Thursday, 30 September 2021 5:34:05 AM AEST Jason Gunthorpe wrote:

On Wed, Sep 29, 2021 at 12:50:15PM +0100, Joao Martins wrote:

quoted

quoted

If the get_dev_pagemap has to remain then it just means we have to
flush before changing pagemap pointers

Right -- I don't think we should need it as that discussion on the other
thread goes.

OTOH, using @pgmap might be useful to unblock gup-fast FOLL_LONGTERM
for certain devmap types[0] (like MEMORY_DEVICE_GENERIC [device-dax]
can support it but not MEMORY_DEVICE_FSDAX [fsdax]).

When looking at Logan's patches I think it is pretty clear to me that
page->pgmap must never be a dangling pointer if the caller has a
legitimate refcount on the page.

For instance the migrate and stuff all blindly calls
is_device_private_page() on the struct page expecting a valid
page->pgmap.

This also looks like it is happening, ie

void __put_page(struct page *page)
{
	if (is_zone_device_page(page)) {
		put_dev_pagemap(page->pgmap);

Is indeed putting the pgmap ref back when the page becomes ungettable.

This properly happens when the page refcount goes to zero and so it
should fully interlock with __page_cache_add_speculative():

	if (unlikely(!page_ref_add_unless(page, count, 0))) {

Thus, in gup.c, if we succeed at try_grab_compound_head() then
page->pgmap is a stable pointer with a valid refcount.

So, all the external pgmap stuff in gup.c is completely pointless.
try_grab_compound_head() provides us with an equivalent protection at
lower cost. Remember gup.c doesn't deref the pgmap at all.

Dan/Alistair/Felix do you see any hole in that argument??

As background note that device pages are currently considered free when
refcount == 1 but the pgmap reference is dropped when the refcount transitions
1->0. The final pgmap reference is typically dropped when a driver calls
memunmap_pages() and put_page() drops the last page reference:

void memunmap_pages(struct dev_pagemap *pgmap)
{
        unsigned long pfn;
        int i;

        dev_pagemap_kill(pgmap);
        for (i = 0; i < pgmap->nr_range; i++)
                for_each_device_pfn(pfn, pgmap, i)
                        put_page(pfn_to_page(pfn));
        dev_pagemap_cleanup(pgmap);

If there are still pgmap references dev_pagemap_cleanup(pgmap) will block until
the final reference is dropped. So I think your argument holds at least for
DEVICE_PRIVATE and DEVICE_GENERIC. DEVICE_FS_DAX defines it's own pagemap
cleanup but I can't see why the same argument wouldn't hold there - if a page
has a valid refcount it must have a reference on the pagemap too.

 - Alistair

So lets just delete it!

Jason